1. Field of the Invention
The present invention relates to a drive circuit of an active-matrix type liquid crystal display (LCD), which supplies a picture signal to the LCD so as to drive the liquid crystal elements.
2. Description of the Related Art
Generally, displays are the most important electronic devices for interconnecting machines and humans. That is, displays communicate various visual data to humans via characters and images. In particular, after personal computers appeared, displays became indispensable electric devices and have been improved so as to realize more convenient display forms for humans.
Recently, LCDs are widely used as displays for notebook-sized personal computers or portable information devices. This is because LCDs are thinner and lighter than CRT (cathode-ray tube) displays. Therefore, in personal computers, it is important to satisfactorily display multi-media data (in particular, image data) obtained via the Internet or the like; therefore, the quality of images shown on LCDs is important.
In addition, LCDs are also widely used as displays of liquid crystal televisions or used as viewfinders of video cameras; therefore, liquid crystal elements must be driven for sufficiently representing the gradation data of each picture signal.
FIG. 8 shows a conventional drive circuit of an active-matrix type LCD. In the figure, in order to improve the quality of the images as explained above, gamma correction circuit 1 is provided for correcting the linearity of the gradations (of gradation data of each input picture signal) with respect to the quantity of light. In addition, in order to improve the quality of the images displayed on LCD 4, the drive circuit employs means for improving the S/N ratio of each of gamma correction circuit 1, polarity inversion circuit 33, and video amplifier 34, and for increasing the dynamic range of the video amplifier 34.
However, in the above-explained drive circuit of a conventional LCD, an image signal having a voltage level over each dynamic range of the gamma correction circuit 1, polarity inversion circuit 33, and video amplifier 34 may be input into those circuits. If an image signal having a voltage level over the dynamic range of the video amplifier 34 is input, then the video amplifier 34 does not normally operate and does not output normal picture signals, and a ghost or the like appears on the display screen of the LCD, thereby degrading the quality of the displayed image. On the other hand, if an image signal having a voltage level over the dynamic range of the polarity inversion circuit 33 is input, then the following video amplifier 34 does not output normal picture signals, and a ghost or the like appears on the display screen of the LCD, thereby degrading the quality of the displayed image.
In consideration of the above circumstances, an objective of the present invention is to provide a drive circuit of an LCD, by which no image signal having a voltage level over the dynamic range of each circuit element is input, and thus erroneous operation of each circuit element is prevented, thereby supplying normal image signals to the LCD.
Therefore, the present invention provides a drive circuit of an active-matrix type liquid crystal display (LCD), for supplying a picture signal, whose polarity is alternately changed like an alternating current, to each electrode of liquid crystal elements of the LCD, comprising:
a clip circuit for clipping the amplitude range of the voltage of the picture signal input from an input terminal;
a polarity inversion circuit for receiving the picture signal whose amplitude range was clipped by the clip circuit, and for converting the picture signal so that an inverted signal and a non-inverted signal are alternately assigned to each dot; and
a video amplifier for amplifying the voltage level of the converted picture signal by a predetermined amplification degree.
Typically, the drive circuit clips the amplitude range of the voltage of the picture signal in a manner such that the clipped amplitude range is suitable for the dynamic range of the video amplifier and the output voltage level from the video amplifier does not exceed the relevant dynamic range.
Preferably, the upper limit voltage and the lower limit voltage of the amplitude range in the clipping operation executed by the clip circuit are variable according to the voltage of a control signal supplied to the clip circuit.
The clip circuit may comprise two serially connected transistors, and predetermined control voltages are respectively applied to the bases of the two transistors, and the contact between the two transistors may be connected to an input terminal of the polarity inversion circuit.
In this case, the upper limit voltage and the lower limit voltage of the amplitude range in the clipping operation executed by the clip circuit are variable according to the control voltages.
The clip circuit may comprise two serially connected diodes, and predetermined control voltages are respectively applied to the cathode of one of the two diodes and the anode of the other diode, and the contact between the two diodes is connected to an input terminal of the polarity inversion circuit.
In this case, the upper limit voltage and the lower limit voltage of the amplitude range in the clipping operation executed by the clip circuit are variable according to the control voltages.
The drive circuit may further comprise:
a gamma correction circuit for correcting the gradation characteristics of the picture signal, and wherein:
the clip circuit is connected to an input terminal of the gamma correction circuit and an output terminal of the gamma correction circuit is connected to an input terminal of the polarity inversion circuit.
Preferably, the drive circuit clips the amplitude range of the voltage of the picture signal in a manner such that the clipped amplitude range is suitable for the dynamic range of the gamma correction circuit and the output voltage level from the gamma correction circuit does not exceed the relevant dynamic range.
Also preferably, the drive circuit clips the amplitude range of the voltage of the picture signal in a manner such that the clipped amplitude range is suitable for the dynamic range of the polarity inversion circuit and the output voltage level from the polarity inversion circuit does not exceed the relevant dynamic range.
According to the present invention, the clip circuit can clip the amplitude range of the input picture signal corresponding to the dynamic range of the video amplifier; thus, it is possible to prevent a picture signal having a voltage level which exceeds the dynamic range of the video amplifier from inputting. Therefore, an erroneous operation of the video amplifier can be prevented, and normal picture signals can be continuously output from the video amplifier, thereby improving the quality of images displayed on the screen of the LCD.
In addition, the clip circuit can also clip the amplitude range of the input picture signal corresponding to the dynamic range(s) of the video amplifier and the polarity inversion circuit and/or the gamma correction circuit. Therefore, also in the polarity inversion circuit, it is possible to prevent a picture signal having a voltage level which exceeds the dynamic range of the polarity inversion circuit from inputting. Therefore, normal picture signals can be continuously output from the polarity inversion circuit, and no undesirable effect is imposed on the following video amplifier. Accordingly, normal picture signals can be continuously output from the video amplifier, thereby improving the quality of images displayed on the screen of the LCD.